(1) Field of the Invention
The present invention relates to a 277V lamp having a light-emitting unit and a lighting circuit that is for lighting the light-emitting unit.
(2) Description of the Related Art
Conventional low-pressure discharge lamps have an arc tube and an electronic lighting circuit for lighting the arc tube. Examples of such lamps include self-ballasted fluorescent lamps (hereinafter, also referred to simply as “lamps”). There are two types of lamps depending on the rated lamp voltage. One is a 277V type designed for operation at 277 volts. The other is a 120V type designed for operation at 120 volts. (Hereinafter 277V type and 120V type lamps are simply referred to as “277V lamps” and “120V lamps”, respectively.) Mainly, 277V lamps are for use at commercial facilities and outdoors, whereas 120V lamps are for use at home.
Lamps of the both types are similar in size and shape. Thus, it is often the case that a 277V lamp is mistakenly used with a 120V lighting fixture (Such operation is improper and is also referred to as “misuse”).
FIGS. 1 and 2 are circuit diagrams of lighting units employed in conventional lamps.
A lighting unit 901 illustrated in FIG. 1 includes a rectifier circuit 903 for rectifying commercial low-frequency current voltage into direct current voltage, a smoothing circuit 905 for smoothing the rectified direct current voltage, an inverter circuit 909 for applying a high-frequency voltage to an arc tube 907. The rectifier circuit 903 is composed of so-called a bridge diode, whereas the smoothing circuit 905 is composed of a smoothing capacitor 9C2.
The inverter circuit 909 includes two switching elements 9Q1 and 9Q2 and applies a high-frequency voltage to the arc tube 907, by alternately switching ON and OFF the two switching elements 9Q1 and 9Q2. With application of the high-frequency voltage, the arc tube 907 emits light, so that high frequency power starts to be supplied to the arc tube 907.
The lighting unit 901 illustrated in FIG. 1 is connected to a commercial power source via a base 902. Upon startup of the lamp operation, the electric current output from the smoothing circuit 905 flows through the resistors 9R1 and 9R2 and then the capacitor 9C1. When the charged voltage of the capacitor 9C1 reaches a predetermined value, a trigger diode 9TD2 breaks down and turns ON the switching element 9Q2.
Similarly to the lighting unit 901 illustrated in FIG. 1, a lighting unit 911 illustrated in FIG. 2 includes a rectifier circuit 913, a smoothing circuit 915, and an inverter circuit 917.
Upon startup of the lamp operation, the electric current output from the rectifying and smoothing circuits flows through the resistors 9R3, 9R4, and 9R5. When the fraction of voltage obtained by the resistor 9R4 reaches a predetermined value, the switching element 9Q3 is turned ON.
Although a 277V lamp and a 120V lamp differ in their rated lamp voltages, the respective lighting units are normally the same in circuitry. For this reason, even if a 277V lamp is mistakenly used for a 120V lamp, the switching elements 9Q1 and 9Q2 still start to cause the lamp to emit light. In this case of misuse, the lamp manages to operate without flickering but with various problems. For example, the intensity of a 277V lamp mistakenly used for a 120V lamp is slightly lower than the intensity of a 120V lamp under normal operation. In addition, the lamp life is shorter than the rated lamp life.
Since a mistakenly used 277V lamp operates without flickering and other immediately noticeable problems, the user may not be able to recognize that the 277V lamp is improperly used in place of a 120V lamp. With this being a situation, there is a risk that the user has a wrong impression that the lower lamp intensity and shorter lamp life are simply due to the bad quality of the lamp.